1. Technical Field
This disclosure relates to switching converters and more particularly to a method of controlling a switching converter in burst mode, a controller for implementing the method and a related switching converter.
2. Description of the Related Art
Switching converters are widely used in the field of power supply devices because of their performances in supplying high loads with a regulated voltage or current. They are characterized by a high switching frequency and, for reducing potentially large periodic losses at turn ON and OFF of the switches, numerous converters use a resonant tank circuit to reduce switching losses through zero-voltage switching or zero-current switching techniques.
A basic fed-back architecture of a switching converter is depicted in FIG. 1. It has a forward path and a feedback path, the forward path comprising:                a switching stage SS receiving in input an unregulated voltage VIN,        an energy tank circuit TC, that may be for example an LLC circuit, or a LCC circuit, or a LC circuit, configured to be driven by the switching stage SS,        an optional isolation interface IB, that may be for example a transformer,        a rectifier REC of the oscillating voltage made available by the isolation interface IB, and        a low-pass filter LPF that generates a regulated DC output voltage or current Σout delivered to a supplied load RL;        
the feedback path comprising:                an adder that generates a difference signal ε between a nominal output voltage or current Σref and the regulated DC output voltage or current Σout,        an error amplifier EA provided with a compensation network CN, that generates an error signal Vcomp representative of the difference between the regulated DC voltage or current Σout and the respective nominal value Σref, and        a switch controller SC that generates a control signal σ the level of which is determined upon the error signal Vcomp for fixing the working frequency (frequency control), the intensity of the delivered output current (peak current mode control), or the on-times (time shift control), etc.        
The control signal σ ranges between a minimum and a maximum level, fixed for controlling the switching stage SS such to inject a minimum current in the energy tank circuit TC sufficient to ensure a Zero-Voltage Switching (ZVS), or to prevent excessive switching frequencies as well as to prevent too large variations of the functioning frequency, etc.
Switching converters show good performances when heavy loads are supplied, though their efficiency worsens when light loads are supplied, unless they are not properly managed. For this reason, when the supplied load RL is light, that is when the control signal σ has attained its minimum value and cannot be reduced further, switching converters are operated in burst mode by a feedback circuit as shown in FIG. 2. When the error signal Vcomp becomes smaller than a burst-stop threshold Vburst, that means that the supplied load RL has reduced, a first comparator COMP1 generates an active stop signal STOP that makes the switch controller SC disable the switching of the switching stage SS; when the error signal Vcomp becomes greater than a burst-enable threshold Vrun, that means that the output voltage or current Σout has decreased, a second comparator COMP2 generates an active enable signal START that makes the switch controller SC enable the switching of the switching stage SS. FIG. 3 depicts a time graph of the signals of the switching converter of FIG. 2 that illustrates a burst mode functioning.
The graph of FIG. 3 shows that, when in burst mode, the switching stage SS is enabled for certain time intervals, represented in FIG. 3 in gray, that occur periodically at a burst frequency Fburst. The so-called “duty ratio”, that is the ratio between the duration of enabling intervals during which the switching stage is enabled and the burst period 1/Fburst with which these enabling intervals occur, corresponds to the fraction of the minimum power, that may be output by the converter, absorbed by the supplied load.
Typically, the enabling intervals have a fixed duration, as shown in FIG. 3.
This functioning mode causes a ripple of the output voltage or current Σout that may be reduced using an extremely selective (and thus expensive) low-pass filter LPF. In any case, the output ripple in this functioning condition is increased in respect to a functioning condition in which the converter supplies a load heavier than a minimum value corresponding to the burst-stop threshold Vburst. Moreover, very often the burst frequency Fburst is comprised in the range of acoustic frequencies, and thus may cause audible noise.